The conventional apparatus in the market for photomask inspection generally employ ultra-violet (UV) light with wavelengths at or above 193 nanometers (nm). This is suitable for masks designed for use in lithography based on 193 nm light. To further improve the printing of minimum feature sizes, next generation lithographic equipment is now designed for operation in the neighborhood of 13.5 nm. Accordingly, patterned masks designed for operation near 13 nm must be inspected. Such masks are reflective, having a patterned absorber layer over a resonantly-reflecting substrate (EUV multilayer), typically 40 pairs of molybdenum silicide (MoSi) with a 7 nm period. The conventional inspection apparatus uses optics with a combination of wavelength and numerical apertures (NA) that are not sufficient (i.e., too small) to resolve pattern features and pattern defects of interest (printable) in EUV mask patterns characterized by a half-pitch below 22 nm.
As part of the optics in the conventional inspection apparatus, mirrors of varying sizes and shapes are used to receive and reflect EUV light reflected from the substrate. To keep the telecentric condition at the image plane, some mirrors are notably large in size (e.g., 280 mm by 200 mm). The sheer size of the mirror significantly increases the cost of production. Moreover, large mirrors require an increased thickness to reduce self-weight distortion, which adds to the overall cost of production.
Mounting a large mirror in an inspection device requires a robust mounting structure to reduce self-weight distortion and to reduce the static movement of the mirror in rigid body motion due to the large mirror weight deflecting the main structure. The configuration of the mounting structure should protect against distortion or unacceptable motion of a large mirror. In addition, a large mirror may require a graduated or indexed coating to meet distortion needs. Adding a graduated or indexed coating to a large mirror adds complexity to the manufacturing process.
During the mirror manufacturing and inspection process, mirrors are tested to ensure they receive and reflect light as intended. Large mirrors are more expensive and difficult to test. When a large mirror is removed from the mounting structure, technicians must protect the mirror from distorting under its own weight. Moreover, numerous test points are required for large mirrors due to the number of small sub-apertures. This additional testing requires more labor and time to validate the mirror. If the testing uncovers variations or defects in the mirror surface, the mirror must be reworked to fix the variation or defect. Rework can be a continual process, which impacts yield and cost.
Thus, there is a long-felt need for an invention to improve upon the shortcomings of large mirrors that would reduce mirror distortion and improve testing efficiency. More specifically, there is a long-felt need for a segmented mirror apparatus for light imaging and a method of using the same.